The described technology relates to the technical field of switching regulators and, in particular, to a power delivery network that automatically compensates for a load step response produced when the power delivery network is placed under a load condition.
Power delivery networks, such as those in power supplies, may include regulators that switch at frequencies between 100 kHz and 5 MHz. One common type of switching regulator is a current mode regulator. A current mode regulator may include a fast current loop that detects a ramping current through a power switch or through an inductor, and a much slower voltage loop that detects the output voltage. The magnitude of output voltage is dependent upon the magnitude of duty cycle produced by the regulator. A large capacitor connected to the output of the regulator may smooth the output voltage.
A load current generated by the regulator may instantaneously change when a load connected to the regulator goes into or comes out of a standby mode. If, for example, the load comes out of a standby mode, an instantaneous increase in current requirements may draw charge from the output capacitor and cause the output voltage to temporarily droop until the next switching cycle. During the next switching cycle, the regulator may detect the lowered output voltage and increase the duty cycle of the power switch to compensate for the increased load current. Correcting for a current transient is a function of the voltage feedback loop bandwidth, and thus may take several clock cycles to complete, resulting in output voltage ripple. This voltage ripple may cause a regulated voltage to transition out of a desired operating range.
Conventional approaches to reducing output voltage ripple from current transients include increasing the size of the output capacitor (C-OUT) and/or reducing the capacitor ESR (equivalent series resistance). These techniques are costly and require significant board space. What is needed is a technique for reducing output voltage ripple in regulators due to load current transients, without significantly increasing load capacitance.
A switching regulator may also adjust its duty cycle based on a feedback voltage and a reference voltage applied to an error amplifier. The feedback voltage may be adjusted by changing the resistor ratio in a resistor divider connected between the output voltage and the input into the error amplifier. Other adjustments to the feedback voltage may be based on the voltage loop. The reaction time of the regulator based on the voltage loop, however, may be on the order of 30 μS or more for only a 0.1 volt step. This delay may occur whether the voltage step is in a positive or negative direction. On the other hand, increasing the bandwidth of the voltage loop bandwidth may adversely impact the phase margin. What is needed therefore is a technique for reducing the delay in reaching a target regulated voltage in response to an external command to change the regulated output voltage.
Additionally, assuring a proper transient load step response to a current load may be a complicated task for a power supply designer. Testing whether a power supply design provides a proper transient response to a load may include applying a load to the output of the power supply regulator and monitoring the signal response of the system to determine whether the regulator can adequately perform under the load. In one example, a high speed adjustable current load may be applied to VOUT while measuring the transient response on an oscilloscope. Also, determining stability of the system may include injecting a variable frequency sinusoidal signal into the closed loop feedback and measuring the gain and phase relationship to this input over the applied frequency range. However, this technique may require an additional resistor in the feedback loop and a sophisticated measurement system.
The foregoing measurement techniques are typically employed under lab conditions and are not utilized in a production environment or in the field, and producing and measuring a transient current load step response in a power supply regulator is not seen as being applicable to large scale manufacturing testing. Moreover, testing a single design prototype in the laboratory does not guarantee that each manufactured device achieves the same standards. Accordingly, what is needed is a way to test power supply regulators during the manufacturing process to achieve better quality control.